By combining nonpreferential layers with graphoepitaxy techniques, directed assembly of both perpendicular cylinders and lamellae of PS-b-PMMA has been successfully demonstrated. Particularly, in the case of lamella-forming block copolymers (BCPs) it is desirable to align the perpendicular lamellae along the side walls. Previous studies on lamella-forming PS-b-PMMA have mainly focused on the heterogeneous chemical modification of topographic patterns (Fig. 1a), i.e. nonpreferential trench bottoms and preferential sidewalls. We have developed a one step approach that can control the lateral orientation of perpendicular lamellae on a topographic pattern with homogeneous chemical modification (Fig. 1b). Instead of controlling each component namely the interfacial interaction of PS-b-PMMA with the sidewall and the trench bottom independently, we have focused on the thickness dependence of the microdomain orientation. In this study, we chose three different composition of hydroxyl terminated P(S-r-MMA) surface neutralizing copolymers with compositions that are weakly PMMA preferential, weakly PS preferential or non-preferential. To study the vertical confinement effect as a result of BCP film thickness, the BCP film thickness was varied on the random copolymer modified planar substrates. On the non-preferential surface perpendicular lamellae were induced at all thicknesses that were tested, whereas on the weakly preferential surfaces the morphology is thickness dependent and switches to vertical orientation in the middle of quantized thicknesses (Figure 1d). To translate these studies to topographic patterns, we used homogeneous chemical modification along with controlled film thickness to induce perpendicular orientation. For graphoepitaxy shown in Figure 1c, we intentionally avoided commensurable BCP thickness on weakly preferential surfaces to align perpendicular lamellae along the trenches. In summary, we have developed a simple and elegant graphoepitaxy based method which uses a single step homogeneous chemical modification to create perpendicularly oriented lamellar domains aligned along the sidewalls. By simultaneously controlling the thickness of the BCP film and the interfacial interactions we can dictate the right boundary conditions.
Graphoepitaxial Assembly of Symmetric Block Copolymers on Weakly Preferential Substrates. Han, E.; Kang, H.; Liu, C.-C.; Nealey, P. F.; Gopalan, P. Advanced Materials 2010. 22, 4325-4329.